In today's point to point communications, trunked communication systems provide many advantages over conventional systems. The trunked communication systems provide efficient allocation of a limited number of communication channels to a large number of users. Additionally, many system features, such as telephone interconnect, may be easily incorporated into the trunked communication systems.
In a trunked communication system a communication resource controller and a plurality of communication units, such as portable and/or mobile two-way radios, communicate analog messages, i.e. voice and/or tone signalling, and synchronous digital data messages. The digital data messages having predetermined baud rate are communicated on one or more control channels. The analog messages, such as Dual Tone Multiple Frequency (DTMF) tones, are communicated on one or more voice channels.
In a trunked system, an analog communication is preceded by a digital communication. The communication resource controller upon reception of a digital inbound channel request from one of the communication units may allocate a voice channel to that communication unit if such a voice channel is available. The communication resource controller may only receive a digital inbound channel request during a predetermined access time interval. If an inbound channel request is received within the access time interval, a response is transmitted to the communication unit which either indicates allocation of an available channel or indicates that all channels are busy. Once an available channel is allocated, voice and/or tone signalling messages may be communicated on that channel.
Generally, because of error correcting routines digital communication is substantially more reliable than analog communication which lack such routines. Thus, a voice channel may successfully be allocated after the communication resource controller has received a digital channel request message, however, the subsequent analog communication, i.e., voice and tone signalling communication may fail, if the communication unit is situated in a proximity which makes analog communication impossible. The failure to establish analog communication may hamper effective communication and cause user frustration. Therefore, it is desirable to provide successful analog communication after a successful prior digital communication is established.